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solar time
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- Sidereal day (prograde).svg|right|thumb|300px|On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360Â° and the distant star is overhead again (1â†’2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1â†’3 = one solar day). More simply, 1-2 is a complete rotation of the Earth, but because the revolution around the Sun affects the angle at which the Sun is seen from the Earth, 1-3 is how long it takes noonnoonSolar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time (sundial time) and mean solar time (clock time).

Apparent solar time

| 24 hours
| 24 hours âˆ’ 18.1 seconds
| 24 hours
| 24 hours + 13.1 seconds
| 24 hours
| 24 hours âˆ’ 21.3 seconds
| 24 hours
| 24 hours + 29.9 seconds
These lengths will change slightly in a few years and significantly in thousands of years.

Mean solar time

thumb|right|250px|The equation of timeâ€”above the axis a sundial will appear fast relative to a clock showing local mean time, and below the axis a sundial will appear slow.Mean solar time is the hour angle of the mean Sun plus 12 hours. This 12 hour offset comes from the decision to make each day start at midnight for civil purposes whereas the hour angle or the mean sun is measured from the zenith (noon).WEB,weblink Archived copy, 2018-03-28, no,weblink" title="web.archive.org/web/20180328102907weblink">weblink 2018-03-28, Currently (2009) this is realized with the UT1 time scale, constructed mathematically from very long baseline interferometry observations of the diurnal motions of radio sources located in other galaxies, and other observations.McCarthy, D. D. & Seidelmann, P. K. (2009). TIME From Earth Rotation to Atomic Physics. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA. {{ISBN|978-3-527-40780-4}}. pp. 68, 326.Capitaine, N., Wallace, P. T., & McCarthy, D. D. (2003). "Expressions to implement the IAU 2000 definition of UT1" {{webarchive|url=https://web.archive.org/web/20160407013603weblink |date=2016-04-07 }}, Astronomy and Astrophysics, vol.406 (2003), pp.1135-1149 (or in pdf form); and for some earlier definitions of UT1 see Aoki, S., H Kinoshita, H., Guinot, B., Kaplan, G. H., D D McCarthy, D. D., & Seidelmann, P. K. (1982) "The new definition of universal time", Astronomy and Astrophysics, vol.105 (1982), pp. 359-361. The duration of daylight varies during the year but the length of a mean solar day is nearly constant, unlike that of an apparent solar day.For a discussion of the slight changes that affect the mean solar day, see the Î”T article. An apparent solar day can be 20 seconds shorter or 30 seconds longer than a mean solar day."The duration of the true solar day" {{webarchive|url=https://web.archive.org/web/20090826184737weblink |date=2009-08-26 }}. Pierpaolo Ricci. pierpaoloricci.it. (Italy) Long or short days occur in succession, so the difference builds up until mean time is ahead of apparent time by about 14 minutes near February 6 and behind apparent time by about 16 minutes near November 3. The equation of time is this difference, which is cyclical and does not accumulate from year to year.Mean time follows the mean sun. Jean Meeus describes the mean sun as follows:Consider a first fictitious Sun travelling along the ecliptic with a constant speed and coinciding with the true sun at the perigee and apogee (when the Earth is in perihelion and aphelion, respectively). Then consider a second fictitious Sun travelling along the celestial equator at a constant speed and coinciding with the first fictitious Sun at the equinoxes. This second fictitious sun is the mean Sun..."Meeus, J. (1998). Astronomical Algorithms. 2nd ed. Richmond VA: Willmann-Bell. p. 183.The length of the mean solar day is slowly increasing due to the tidal acceleration of the Moon by the Earth and the corresponding slowing of Earth's rotation by the Moon.

History

File:Sun and Moon Nuremberg chronicle.jpg
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Many methods have been used to simulate mean solar time. The earliest were clepsydras or water clocks, used for almost four millennia from as early as the middle of the 2nd millennium BC until the early 2nd millennium. Before the middle of the 1st millennium BC, the water clocks were only adjusted to agree with the apparent solar day, thus were no better than the shadow cast by a gnomon (a vertical pole), except that they could be used at night.But it has long been known that the Sun moves eastward relative to the fixed stars along the ecliptic. Since the middle of the first millennium BC the diurnal rotation of the fixed stars has been used to determine mean solar time, against which clocks were compared to determine their error rate. Babylonian astronomers knew of the equation of time and were correcting for it as well as the different rotation rate of the stars, sidereal time, to obtain a mean solar time much more accurate than their water clocks. This ideal mean solar time has been used ever since then to describe the motions of the planets, Moon, and Sun.Mechanical clocks did not achieve the accuracy of Earth's "star clock" until the beginning of the 20th century. Today's atomic clocks have a much more constant rate than the Earth, but its star clock is still used to determine mean solar time. Since sometime in the late 20th century, Earth's rotation has been defined relative to an ensemble of extra-galactic radio sources and then converted to mean solar time by an adopted ratio. The difference between this calculated mean solar time and Coordinated Universal Time (UTC) determines whether a leap second is needed. (The UTC time scale now runs on SI seconds, and the SI second, when adopted, was already a little shorter than the current value of the second of mean solar time.:(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C Hackman and R A Nelson, in Astronomical Journal, vol.136 (2008), pages 1906-1908, it is stated (page 1908), that "the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens." :(2) In the late 1950s, the cesium standard was used to measure both the current mean length of the second of mean solar time (UT2) (result: 9192631830 cycles) and also the second of ephemeris time (ET) (result:9192631770 Â± 20 cycles), see "Time Scales", by L. Essen {{webarchive|url=https://web.archive.org/web/20081019014533weblink |date=2008-10-19 }}, in Metrologia, vol.4 (1968), pp.161-165, on p.162. As is well known, the 9192631770 figure was chosen for the SI second. L Essen in the same 1968 article (p.162) stated that this "seemed reasonable in view of the variations in UT2".)

References

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{{Time Topics}}{{Time measurement and standards}}{{The Sun}}

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